1. Field of the Invention
The invention concerns pivotal connections which are used primarily for adjustment of the angle of inclination of the backrest part of a seat with respect to the seat part. It is applied in particular to vehicle seats, for example those which are used in motor vehicles.
2. Background of the Related Art
Different types of pivotal connections which involve continuous adjustment are known, in particular those described in French Pat. No. 1.590.551. That document describes three embodiments of continuous pivotal connections comprising a planet gear which cooperates by meshing engagement with two ring gears. One of the ring gears is fixed with respect to the backrest part of the seat by way of a suitable fitting and the other is fixed with respect to the seat part, also by means of another fitting. A control member drives the planet gear by way of a shaft which rotates an eccentric and the planet gear rotates the movable ring gear which is fixed with respect to the backrest part, relative to the fixed ring gear which is fixed with respect to the seat part. That thus provides for adjustment of the inclination of the backrest part.
It is stated in said document that with the use of an epicyclic train means, the continuous pivotal connection described is irreversible, that is to say, it is not possible to cause rotary movement of one of the ring gears with respect to the other, without acting on the control member which actuates the eccentric. That means that if an attempt is made to turn the movable ring gear by acting for example on the backrest part, the pivotal connection is automatically locked. It has been found that two embodiments described in that document are in fact irreversible. The pivotal connections corresponding to those two embodiments have the common characteristic that the planet gear comprises a single tooth array, the teeth of which are of a sufficient width to mesh at the same time with the two tooth arrays of the ring gears which are disposed in side-by-side relationship, in the meshing zone. Thus, a couple, i.e., a rotating torque, which is applied to the movable ring gear, for example by virtue of a thrust applied to the backrest part, creates at the single tooth array of the planet gear a drive couple which is opposed by a resisting couple which is equal and opposite, being produced at that same tooth array of the planet gear by the fixed ring gear.
Experience has shown however that the two pivotal connection constructions comprising a planet gear with a single tooth array suffer from disadvantages. In fact, the first embodiment uses two rings gears which are mounted on a common axis having tooth arrays of the same diameters but with different numbers of teeth which mesh with the single tooth array, and that gives rise to play and wear. In the second embodiment the diameters of the tooth arrays of the ring gears are different but one of the two is driven by an eccentric about the axis of the other in such a way that, in the meshing zone of the planet gear, the single tooth array thereof does in fact co-operate with the two tooth arrays of the ring gears. This substantially increases the degree of complexity of the pivotal connection and also gives rise to increased wear and play. In the third embodiment described in the above-indicated document the two ring gears are mounted on a common axis and have inwardly directed tooth arrays of different diameters. The planet gear has two tooth arrays which are disposed in side-by-side relationship, which are of different diameters and which are directed outwardly, the dimensions thereof being such that, under the action of the eccentric, they simultaneously mesh with the corresponding tooth arrays of the two ring gears.
The last-mentioned embodiment of the continuous pivotal connection for a seat has advantages in terms of simplicity of design in comparison with those described above, but, contrary to what French Pat. No. 1.590.551 states, it is not irreversible if the difference in diameter between the two tooth arrays of the planet gear becomes appreciable. It will be appreciated that the drive torque which is possibly applied by the movable ring gear is in fact no longer balanced by the resisting torque opposed by the fixed ring gear, the lever arms with respect to the common axis being different.
The degree of reversibility of this last-mentioned embodiment increases in proportion to an increasing ratio between the diameters of the two tooth arrays of the planet gear. In practice, it may be an attractive proposition to increase that ratio so as to permit relatively rapid adjustment of the angle of inclination of the backrest part, which does not require a large number of turns of the eccentric. In practice also, in order to reduce the force which has to be applied to the means for actuating the eccentric with a rotary movement, it is often necessary to reduce the amount of friction between the components of the pivotal connection. That is achieved by lubrication and also by using ball-type or roller-type rolling bearings. All those measures increase the degree of reversibility of the pivotal connection and the consequence thereof is that a thrust applied to the backrest part of the seat with a sufficient force causes the planet gear to be driven in rotation with rotary movement of the eccentric about their common axis and angular displacement of the movable ring gear and the backrest part. This is a major disadvantage, in particular in regard to a vehicle seat, as in the event of a violent impact there is the risk of the backrest part pivoting abruptly in one direction or the other. The fact that the assembly can go out of adjustment in that way, which can also be a progressive procedure by virtue of the force applied to the backrest part by a seated person and/or due to vibration from the road, is in itself a nuisance and a possible cause of accidents.